1. Field of the Invention
The present invention relates to the use of cellulose fibers of natural origin for structurally modifying polyvinyl chloride (PVC) articles, such as bands and profiles, and to such articles.
2. Description of the Related Art
The use of cellulose fibers of natural origin as fillers is known in the polymer processing industry. These are highly pure, white powders or fibers having a cellulose content of at least 99.5% with reference to the dry substance. Such cellulose fillers are distinguished by defined bulk weights which ensure optimum incorporation into the basic substance of the composition to be processed. The cellulose fillers are not abrasive and are substantially inert to acids, bases and solvents.
Cellulose filers may be admixed with particular advantage in flowable polymer-based compositions because the manufacturing process employed for such compositions ensures optimum anchorage of the individual fibers which have a fibrillar surface. The fibers are obtained from wood which has been appropriately comminuted and chemically cleaned or lignin and other impurities. Following the cleansing process, the cellulose raw material is ground into a variety of fiber lengths.
In the production of articles made of polyvinyl chloride, particularly band and profile articles, considerable shrinkage is known to occur if these articles are subjected to use at elevated temperatures. In a test at 11.degree. C. for one hour, shrinkage on the order of 2 to 4% of the original length is not unusual. Such shrinkage, particularly a reduction in the length of elongate articles, is a major drawback to use of polyvinyl chloride and renders polyvinyl chloride articles unacceptable for use in many applications. Although the use of special polymer process tools has made it possible to reduce shrinkage to about 1.5%, this shrinkage value is still too high for many applications.
The use of admixed fillers in processing polyvinyl chloride is known. Fillers which have been employed include particulate fillers, such as calcium carbonate and the like. However, the shrinkage behavior of polyvinyl chloride band and profile articles has been improved only somewhat by such fillers.
If wood meal is used as a filler for polyvinyl chloride, a reduction of shrinkage is realized, but impact strength is considerably reduced thereby. Moreover, the surface quality of band and profile articles composed of polyvinyl chloride charged with wood meal as a filler no longer meets the respective product requirements. Further, such bands and profiles are discolored by the wood meal and are therefore subject to use restrictions since discoloration is not tolerated for many applications.
If glass fibers are used as fillers in the production of polyvinyl chloride band and profile articles a great drop in impact strength results and the bands and profiles have an open, rough surface which is not tolerated in many applications. Moreover, a further drawback in the use of glass fibers as filler is that they cause increased wear of the tools used in the manufacture of the filled articles and also in subsequent processing of the articles.